Method for the catalytic gas phase oxidation of acrolein into acrylic acid

ABSTRACT

In a process for the catalytic gas-phase oxidation of acrolein to acrylic acid, the reaction gas starting mixture is passed, with an acrolein loading of &gt;=150 l (s.t.p.)/l.h, over a fixed-bed catalyst which is housed in two spatially successive reaction zones A, B, the reaction zone B being kept at a higher temperature than the reaction zone A.

The present invention relates to a process for the catalytic gas-phaseoxidation of acrolein to acrylic acid, in which a reaction gas startingmixture comprising acrolein, molecular oxygen and at least one inertgas, at least 20% by volume of which consists of molecular nitrogen, andcontaining the molecular oxygen and the acrolein in a molar ratioO₂:C₃H₄O≧0,5 is passed over a fixed-bed catalyst, whose active materialis at least one molybdenum- and vanadium-containing multimetal oxide, insuch a way that the acrolein conversion in a single pass is ≧90 mol %and the associated selectivity of the acrylic acid formation is ≧90 mol%.

The abovementioned process for the catalytic gas-phase oxidation ofacrolein to acrylic acid is generally known (cf. for example EP-A 714700or EP-A 700893 and the literature cited in these publications) and isimportant in particular as the second oxidation stage in the preparationof acrylic acid by two-stage catalytic gas-phase oxidation starting frompropene. Acrylic acid is an important monomer which is used as such orin the form of an alkyl ester for producing polymers suitable, forexample, as adhesives.

The object of any catalytic fixed-bed gas-phase oxidation of acrolein toacrylic acid is in principle to achieve a very high space-time yield(STY) with respect to desired product (in the case of a continuousprocedure, this is the amount of acrylic acid in liters produced perhour and unit volume of the catalyst bed used).

There is therefore a general interest in carrying out the gas-phaseoxidation with a very high loading of the catalyst bed with acrolein(this is understood as meaning the amount of acrolein in liters understandard temperature and pressure conditions (=l (s.t.p.); the volume inliters which the corresponding amount of acrolein would occupy understandard temperature and pressure conditions, i.e. at 25° C. and 1 bar)which is passed as a component of the reaction gas mixture per hourthrough one liter of catalyst bed), without significantly impairing theacrolein conversion occurring during a single pass of the reaction gasstarting mixture through the catalyst bed and the selectivity of theassociated formation of desired product.

The implementation of the abovementioned is adversely affected by thefact that the gas-phase oxidation of acrolein to acrylic acid on the onehand is highly exothermic and on the other hand is accompanied by amultiplicity of possible parallel and secondary reactions.

With increasing acrolein loading of the catalyst bed and implementationof the desired boundary condition of essentially constant acroleinconversion, it must therefore be assumed that, owing to the increasedlocal heat production, the selectivity of the formation of desiredproduct decreases.

The conventional processes for the catalytic gas-phase oxidation ofacrolein to acrylic acid, wherein nitrogen is used as a main componentof the inert diluent gas and in addition a fixed-bed catalyst present ina reaction zone and homogeneous along this reaction zone, i.e. ofchemically uniform composition over the catalyst bed, is employed andthe temperature of the reaction zone is kept at a value constant overthe reaction zone (temperature of a reaction zone is understood here asmeaning the temperature of the catalyst bed present in the reaction zonewhen the process is carried out in the absence of a chemical reaction;if this temperature is not constant within the reaction zone, the termtemperature of a reaction zone means in this case the number average ofthe temperature of the catalyst bed along the reaction zone), thereforelimit the value to be applied for the acrolein loading of the catalystbed to values ≦150 l (s.t.p.) of acrolein/1 of catalyst bed.h (cf. forexample EP-B 714700; there, the maximum acrolein loading used is 120 1(s.t.p.) of acrolein/l.h).

EP-B 253409 and the associated equivalent, EP-B 257565, disclose that,with the use of an inert diluent gas which has a higher molar heatcapacity than molecular nitrogen, the proportion of propene in thereaction gas starting mixture of a two-stage gas-phase catalyticoxidation of propene to acrylic acid can be increased. Nevertheless, inthe two abovementioned publications too, the maximum realized propeneloading of the catalyst bed, and hence essentially automatically also anacrolein loading of the catalyst bed occurring subsequently on directpassage of the product gas mixture of the propene oxidation stage intothe acrolein oxidation stage, are ≦140 1 (s.t.p.) of reactant (propeneor acrolein)/1.h.

Only in EP-A 293224 have acrolein loadings above 150 1 (s.t.p.) ofacrolein/1.h been realized to date. However, this has been achieved atthe expense of a special inert diluent gas to be used, which iscompletely free of molecular nitrogen. The particular disadvantage ofthis diluent gas is that, in contrast to molecular nitrogen, all itscomponents are desired products which have to be at least partlyrecycled to the gas-phase oxidation in an expensive manner during acontinuous process, for reasons of cost-efficiency.

It is an object of the present invention to provide a process, asdefined at the outset, for the catalytic gas-phase oxidation of acroleinto acrylic acid, which ensures a high space-time yield of acrylic acidwithout having the disadvantages of the high-load procedure of the priorart.

We have found that this object is achieved by a process for thecatalytic gas-phase oxidation of acrolein to acrylic acid, in which areaction gas starting mixture comprising acrolein, molecular oxygen andat least one inert gas, at least 20% by volume of which consists ofmolecular nitrogen, and containing the molecular oxygen and the acroleinin a molar ratio O₂:C₃H₄O≧0.5 is passed, at elevated temperatures, overa fixed-bed catalyst, whose active material is at least one molybdenum-and vanadium-containing multimetal oxide, in such a way that theacrolein conversion in a single pass is ≧90 mol % and the associatedselectivity of the acrylic acid formation is ≧90 mol %, wherein

a) the loading of the fixed-bed catalyst where the acrolein contained inthe reaction gas starting mixture is ≧150 1 (s.t.p.) of acrolein per lof catalyst bed per h,

b) the fixed-bed catalyst consists of a catalyst bed arranged in twospatially successive reaction zones A, B, the temperature of thereaction zone A being from 230 to 270° C. and the temperature of thereaction zone B being from 250 to 300° C. and at the same time being atleast 5° C. above the temperature of the reaction zone A,

c) the reaction gas starting mixture flows first through the reactionzone A and then through the reaction zone B and

d) the reaction zone A extends to an acrolein conversion of from 55 to85 mol %.

Preferably, the reaction zone A extends to an acrolein conversion offrom 65 to 80 mol %. In addition, the temperature of the reaction zone Ais advantageously from 245 to 260° C. The temperature of the reactionzone B is preferably at least 10° C., particularly advantageously 20°C., above the temperature of the reaction zone A and is advantageouslyfrom 265 to 285° C.

The higher the chosen acrolein loading of the catalyst bed in the novelprocess, the greater should be the chosen difference between thetemperature of the reaction zone A and the temperature of the reactionzone B. Usually, however, the abovementioned temperature difference inthe novel process will be not more than 40° C., i.e. the differencebetween the temperature of the reaction zone A and the temperature ofthe reaction zone B can, according to the invention, be up to 15° C., upto 25° C., up to 30° C., up to 35° C. or up to 40° C.

Furthermore, in the novel process, the acrolein conversion based on thesingle pass may be ≧92 mol % or ≧94 mol % or ≧96 mol % or ≧98 mol % andfrequently even ≧99 mol %. The selectivity of the formation of desiredproduct is as a rule ≧92 mol % or ≧94 mol %, frequently ≧95 mol % or ≧96mol % or ≧97 mol %, respectively.

Surprisingly, the abovementioned applies not only in the case ofacrolein loadings of the catalyst bed of ≧150 l (s.t.p.)/l.h or of ≧160l (s.t.p.)/l.h or ≧170 l (s.t.p.)/l.h or ≧175 l (s.t.p.)/l.h or ≧180 l(s.t.p.)/l.h, but also in the case of acrolein loadings of the catalystbed of ≧185 l (s.t.p.)/l.h or of ≧190 l (s.t.p.)/l.h or ≧200 l(s.t.p.)/l.h or ≧210 l (s.t.p.)/l.h and in the case of loadings of ≧220l (s.t.p.)/l.h or ≧230 l (s.t.p.)/l.h or ≧240 l (s.t.p.)/l.h or ≧250 l(s.t.p.)/l.h.

It is surprising that the abovementioned values are achievable even whenthe inert gas used according to the invention comprises ≧30% by volumeor ≧40% by volume or ≧50% by volume or ≧60% by volume or ≧70% by volumeor ≧80% by volume or ≧90% by volume or ≧95% by volume of molecularnitrogen.

Expediently, the inert diluent gas in the novel process comprises from 5to 20% by weight of H₂O and from 70 to 90% by volume of N₂.

Apart from the components stated in this publication, the reaction gasstarting mixture usually contains essentially no further components.

At acrolein loadings above 250 l (s.t.p.)/l.h, the presence of inert(inert diluent gases should in general be those which are converted toan extent of less than 5%, preferably less than 2%, in a single pass)diluent gases, such as propane, ethane, methane, butane, pentane, CO₂,CO, steam and/or noble gases, is recommended for the novel process.However, these gases can of course also be present in the case of lowerloadings. It is also possible to use an inert gas consisting only of oneor more of the abovementioned gases. It is also surprising that thenovel process can be carried out using a catalyst bed which ishomogeneous, i.e. chemically uniform, over both reaction zones, withoutsuffering significant declines in conversion and/or in selectivity.

In the novel process, the acrolein loading usually does not exceed 600 l(s.t.p.)/l.h. Typically, the acrolein loadings in the novel processwithout significant loss of conversion and selectivity are ≦300,frequently ≦250, l (s.t.p.)/l.h.

The operating pressure in the novel process may be either belowatmospheric pressure (for example down to 0.5 bar) or above atmosphericpressure. Typically, the operating pressure will be from 1 to 5,frequently from 1 to 3, bar. The reaction pressure usually will notexceed 100 bar.

According to the invention, the molar ratio of O₂:acrolein in thereaction gas starting mixture must be ≧0.5. Often it is ≧1. Usually,this ratio is ≦3. According to the invention, the molar ratio ofO₂:acrolein in the reaction gas starting mixture is frequently from 1 to2 or from 1 to 1.5.

A suitable source of the molecular oxygen required in the novel processis air, as well as air depleted in molecular nitrogen (e.g. ≧90% byvolume of O₂, ≦10% by volume of N₂).

According to the invention, the amount of acrolein in the reaction gasstarting mixture may be, for example, from 3 to 15, frequently from 4 to10% by volume or from 5 to 8% by volume (based in each case on the totalvolume).

Frequently, the novel process is carried out at anacrolein:oxygen:steam:inert gas volume ratio (l (s.t.p.)) of 1:(0.5 or 1to 3):(0 to 20):(3 to 30), preferably of 1:(1 to 3):(0.5 to 10):(7 to8).

Acrolein produced by catalytic gas-phase oxidation of propene is usuallyused in the novel process. As a rule, the acrolein-containing reactiongases of this propene oxidation are used without intermediatepurification, and it is for this reason that the novel reaction gasstarting mixture can also contain small amounts of, for example,unconverted propene or of byproducts of the propene oxidation. Theoxygen required for the acrolein oxidation must usually also be added tothe product gas mixture of the propene oxidation.

Such a catalytic gas-phase oxidation of propene to acrolein prior to thenovel process is advantageously carried out, analogously to the novelprocess, so that a reaction gas starting mixture comprising propene,molecular oxygen and at least one inert gas, at least 20% by volume ofwhich consists of molecular nitrogen, and containing the molecularoxygen and the propene in a molar ratio O₂:C₃H₆ of ≧1 is passed, atelevated temperatures, over a fixed-bed catalyst whose active materialis at least one molybdenum- and/or tungsten- and bismuth-, tellurium-,antimony-, tin- and/or copper-containing multimetal oxide, in such a waythat the propene conversion during a single pass is ≧90 mol % and theassociated selectivity of the acrolein formation and of the acrylic acidbyproduct formation together is ≧90 mol %, in which oxidation

a) the loading of the fixed-bed catalyst with the propene contained inthe reaction gas starting mixture is ≧160 l (s.t.p.) of propene per l ofcatalyst bed per h,

b) the fixed-bed catalyst consists of a catalyst bed arranged in twospatially successive reaction zones A′, B′, the temperature of thereaction zone A′ being from 300 to 330° C. and the temperature of thereaction zone B being from 300 to 365° C. and at the same time being atleast 5° C. above the temperature of the reaction zone A′,

c) the reaction gas starting mixture flows first through the reactionzone A′ and then through the reaction zone B′ and

d) the reaction zone A′ extends to a propene conversion of from 40 to 80mol %.

Particularly suitable catalysts for the abovementioned gas-phasecatalytic propene oxidation are those of EP-A 15565, EP-A 575897, DE-A19746210 and DE-A 19855913.

Suitable fixed-bed catalysts for the novel gas-phase catalytic acroleinoxidation are all those whose active material is at least one Mo- andV-containing multimetal oxide. Such suitable multimetal oxide catalystsare disclosed, for example, in U.S. Pat. Nos. 3,775,474, 3,954,855,3,893,951 and 4,339,355. Also particularly suitable are the multimetaloxide materials of EP-A 427508, DE-A 2909671, DE-A 3151805, DE-B2626887, DE-A 4302991, EP-A 700893, EP-A 714700 and DE-A 19736105.

Particularly preferred in this context are the exemplary embodiments ofEP-A 714700 and DE-A 19736105.

A multiplicity of the multimetal oxide active materials suitableaccording to the invention can be described by the formula I

Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)  (I),

where

X¹ is W, Nb, Ta, Cr and/or Ce,

X² is Cu, Ni, Co, Fe, Mn and/or Zn,

X³ is Sb and/or Bi,

X⁴ is one or more alkali metals,

X⁵ is one or more alkaline earth metals,

X⁶ is Si, Al, Ti and/or Zr,

a is from 1 to 6,

b is from 0.2 to 4,

c is from 0.5 to 18,

d is from 0 to 40,

e is from 0 to 2,

f is from 0 to 4,

g is from 0 to 40 and

n is a number which is determined by the valency and frequency of theelements in I other than oxygen.

Preferred embodiments among the active multimetal oxides I are thosewhich are covered by the following meanings of the variables of theformula I:

X¹ is W, Nb and/or Cr,

X² is Cu, Ni, Co and/or Fe,

X³ is Sb,

X⁴ is Na and/or K,

X⁵ is Ca, Sr and/or Ba,

X⁶ is Si, Al and/or Ti,

a is from 2.5 to 5,

b is from 0.5 to 2,

c is from 0.5 to 3,

d is from 0 to 2,

e is from 0 to 0.2,

f is from 0 to 1 and

n is a number which is determined by the valency and frequency of theelements I other than oxygen.

Very particularly preferred multimetal oxides I are, however, those ofthe formula I′

Mo₁₂V_(a),Y¹ _(b),Y² _(c),Y⁵ _(f),Y⁶ _(g),O_(n),  (I′)

where

Y¹ is W and/or Nb,

Y² is Cu and/or Ni,

Y⁵ is Ca and/or Sr,

Y⁶ is Si and/or Al,

a′ is from 2 to 4,

b′ is from 1 to 1.5,

c′ is from 1 to 3,

f′ is from 0 to 0.5,

g′ is from 0 to 8 and

n′ is a number which is determined by the valency and frequency of theelements in I′ other than oxygen.

The multimetal oxide active materials (I) suitable according to theinvention are obtainable in a known manner and disclosed, for example,in DE-A 4335973 or in EP-A 714700.

In principle, multimetal oxide active materials suitable according tothe invention, in particular those of the formula I, can be prepared ina simple manner by producing, from suitable sources of the elementalconstituents, a very intimate, preferably finely divided dry blendhaving a composition corresponding to their stoichiometry and calciningsaid dry blend at from 350 to 600° C. The calcination can be carried outeither under inert gas or under an oxidizing atmosphere, e.g. air(mixture of inert gas and oxygen) or under a reducing atmosphere (forexample mixtures of inert gas and reducing gases, such as H₂, NH3, CO,methane and/or acrolein or said reducing gases by themselves). Theduration of calcination may be from a few minutes to a few hours andusually decreases with the temperature. Suitable sources of theelemental constituents of the multimetal oxide active materials I arethose compounds which are already oxides and/or those compounds whichcan be converted into oxides by heating, at least in the presence ofoxygen.

The intimate mixing of the starting compounds for the preparation ofmultimetal oxide materials I can be carried out in dry or in wet form.If it is carried out in dry form, the starting compounds are expedientlyused in the form of finely divided powder and, after mixing and anycompaction, are subjected to calcination. However, the intimate mixingis preferably carried out in wet form.

Usually, the starting compounds are mixed with one another in the formof an aqueous solution and/or suspension. Particularly intimate dryblends are obtained in the mixing process described when exclusivelydissolved sources of the elemental constituents are used as startingmaterials. A preferably used solvent is water. The aqueous materialobtained is then dried, the drying process preferably being carried outby spray-drying the aqueous mixture at outlet temperatures of from 100to 150° C.

The multimetal oxide materials suitable according to the invention, inparticular those of the formula I, can be used for the novel processboth in powder form and after shaping to specific catalyst geometries,where the shaping may be effected before or after the final calcination.For example, unsupported catalysts can be prepared from the powder formfor the active material or its uncalcined precursor material bycompaction to give the desired catalyst geometry (for example bypelleting, or extrusion), it being possible, if required, to addassistants, such as graphite or stearic acid as lubricants and/ormolding assistants and reinforcing agents, such as microfibers of glass,asbestos, silicon carbide or potassium titanate. Suitable geometries ofunsupported catalysts are, for example, solid cylinders or hollowcylinders having an external diameter and a length of from 2 to 10 mm.In the case of the hollow cylinders, a wall thickness of from 1 to 3 mmis expedient. The unsupported catalyst can of course also have sphericalgeometry, it being possible for the sphere diameter to be from 2 to 10mm.

The pulverulent active material or its pulverulent, still uncalcinedprecursor material can of course also be shaped by application topremolded inert catalyst carriers. The coating of the carriers for thepreparation of the coated catalysts is carried out, as a rule, in asuitable rotatable container, as disclosed, for example, in DE-A2909671, EP-A 293859 or EP-A 714700.

Expediently, for coating the carriers, the powder material to be appliedis moistened and, after the application, is dried again, for example bymeans of hot air. The layer thickness of the powder material applied tothe carrier is expediently chosen to be from 10 to 1000 μm, preferablyfrom 50 to 500 μm, particularly preferably from 150 to 250 μm.

Conventional porous or nonporous aluminas, silica, thorium dioxide,zirconium dioxide, silicon carbide or silicates, such as magnesiumsilicate or aluminum silicate, can be used as carrier materials. Thecarriers may have a regular or irregular shape, those having a regularshape and substantial surface roughness, for example spheres or hollowcylinders, being preferred. The use of essentially nonporous, sphericalsteatite carriers which have a rough surface and whose diameter is from1 to 8 mm, preferably from 4 to 5 mm, is preferred. However, the use ofcylinders whose length is from 2 to 10 mm and whose external diameter isfrom 4 to 10 mm as carriers is also suitable. Where rings suitableaccording to the invention are used as carriers, the wall thickness ismoreover usually from 1 to 4 mm. Annular carriers preferably to be usedaccording to the invention have a length of from 3 to 6 mm, an externaldiameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Ringsmeasuring 7 mm x 3 mm x 4 mm (external diameter x length x internaldiameter) are also particularly suitable according to the invention ascarriers. The fineness of the catalytically active oxide materials to beapplied to the surface of the carrier is of course adapted to thedesired coat thickness (cf. EP-A 714700).

Advantageous multimetal oxide active materials to be used according tothe invention are furthermore materials of the formula II,

[D]_(p)[E]_(q)  (II),

where:

D is Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶ _(g″)O_(x″),

E is Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″),

Z¹ is W, Nb, Ta, Cr and/or Ce,

Z² is Cu, Ni, Co, Fe, Mn and/or Zn,

Z³ is Sb and/or Bi,

Z⁴ is Li, Na, K, Rb, Cs and/or H,

Z⁵ is Mg, Ca, Sr and/or Ba,

Z⁶ is Si, Al, Ti and/or Zr,

Z⁷ is Mo, W, V, Nb and/or Ta,

a″ is from 1 to 8,

b″ is from 0.2 to 5,

c″ is from 0 to 23,

d″ is from 0 to 50,

e″ is from 0 to 2,

f″ is from 0 to 5,

g″ is from 0 to 50,

h″ is from 4 to 30,

i″ is from 0 to 20 and

x″,y″ are numbers which are determined by the valency and frequency ofthe elements in II other than oxygen and

p,q are numbers other than zero, whose ratio p/q is from 160:1 to 1:1,

which are obtainable by separately preforming a multimetal oxidematerial E

Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″)  (E),

in finely divided form (starting material 1) and then incorporating thepreformed solid starting material 1 into an aqueous solution, an aqueoussuspension or a finely divided dry blend of sources of the elements Mo,V, Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶, which contains the abovementioned elementsin the stoichiometry D

Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶ _(g″)  (D),

(starting material 2), in the desired ratio p:q, drying any resultingaqueous mixture and calcining the resulting dry precursor materialbefore or after its drying to the desired catalyst geometry at from 250to 600° C.

Preferred multimetal oxide materials II are those in which theincorporation of the preformed solid starting material 1 into an aqueousstarting material 2 is effected at ≦70° C. A detailed description of thepreparation of catalysts comprising multimetal oxide materials II iscontained, for example, in EP-A 668104, DE-A 19736105 and DE-A 19528646.

Regarding the shaping of catalysts comprising multimetal oxide materialsII, the statements made in connection with the catalysts comprisingmultimetal oxide materials I are applicable.

The novel process is carried out in a technically expedient manner in atwo-zone tube-bundle reactor. A preferred variant of a two-zonetube-bundle reactor which may be used according to the invention isdisclosed in DE-C 2830765. However, the two-zone tube-bundle reactorsdisclosed in DE-C 2513405, U.S. Pat. No. 3,147,084, DE-A 2201528 andDE-A 2903582 are also suitable for carrying out the novel process.

This means that, in a simple procedure, the fixed-bed catalyst to beused according to the invention is present in the metal tubes of atube-bundle reactor, and two heating media, as a rule salt melts,essentially spatially separate from one another are passed around themetal tubes. The tube section over which the respective salt bathextends represents, according to the invention, a reaction zone.

This means that, in a simple procedure, a salt bath A flows around thosesections of the tubes (the reaction zone A), in which the oxidativeconversion of the acrolein (in a single pass) takes place until aconversion of from 55 to 85 mol % is reached and a salt bath B flowsaround that section of the tubes (the reaction zone B) in which thesubsequent oxidative conversion of the acrolein (in a single pass) takesplace until a conversion of at least 90 mol % is achieved (if required,further reaction zones which are kept at individual temperature mayfollow the reaction zones A, B to be used according to the invention).

It is technically expedient if the novel process comprises no furtherreaction zones, i.e. the salt bath B expediently flows around thatsection of the tubes in which the subsequent oxidative conversion of theacrolein (in a single pass) takes place to a conversion of ≧92 mol % or≧94 mol % or ≧96 mol % or ≧98 mol % and frequently even ≧99 mol % ormore.

Usually, the beginning of the reaction zone B is behind the maximum hotspot of the reaction zone A. The temperature of the maximum hot spot ofthe reaction zone B is usually below the temperature of the maximum hotspot of the reaction zone A.

According to the invention, the two salt baths A, B can be passedcocurrent or countercurrent through the space surrounding the reactiontubes, relative to the direction of flow of the reaction mixture flowingthrough the reaction tubes. According to the invention, it is of coursealso possible to use cocurrent flow in the reaction zone A andcountercurrent flow in the reaction zone B (or vice versa).

In all the abovementioned configurations within the respective reactionzone, it is of course possible also to superpose a transverse flow onthe flow of the salt melt which is parallel to the reaction tubes, sothat the individual reaction zone corresponds to a tube-bundle reactoras described in EP-A 700714 or in EP-A 700893, and overall a meanderingflow of the heat exchange medium results in the longitudinal sectionthrough the catalyst tube bundle.

Expediently, the reaction gas starting mixture is preheated to thereaction temperature before being fed to the catalyst bed.

In the abovementioned tube-bundle reactors, the catalyst tubes areusually made of ferritic steel and typically have a wall thickness offrom 1 to 3 mm. Their internal diameter is as a rule from 20 to 30 mm,frequently from 22 to 26 mm. It is technically expedient if the numberof catalyst tubes housed in the tube bundle container is at least 5000,preferably at least 10,000. Frequently, the number of catalyst tubeshoused in the reaction container is from 15,000 to 30,000. Tube-bundlereactors having more than 40,000 catalyst tubes tend to be theexception. Inside the container, the catalyst tubes are usuallyhomogeneously distributed, the distribution expediently being chosen sothat the distance between the central inner axes of catalyst tubesclosest to one another (the catalyst tube spacing) is from 35 to 45 mm(cf. EP-B 468290).

Particularly suitable heat exchange media are fluid heating media. Theuse of melts of salts such as potassium nitrate, potassium nitrite,sodium nitrite and/or sodium nitrate, or of metals having a low meltingpoint, such as sodium, mercury or alloys of various metals, isparticularly advantageous.

In all the abovementioned configurations of the flow in the two-zonetube-bundle reactors, the flow rate inside the two required circulationsof heat exchange media is as a rule chosen so that the temperature ofthe heat exchange medium increases by from 0 to 15° C. from the entryinto the reaction zone to the exit out of the reaction zone, i.e.,according to the invention, the abovementioned ΔT may be from 1 to 10°C. or from 2 to 8° C. or from 3 to 6° C.

The temperature at which the heat exchange medium enters the reactionzone A is, according to the invention, usually from 230 to 270° C. Thetemperature at which the heat exchange medium enters the reaction zone Bis, according to the invention, on the one hand usually from 250° C. to300° C. and on the other hand is simultaneously at least 5° C. above thetemperature of the heat exchange medium entering the reaction zone A.

Preferably, the temperature at which the heat exchange medium enters thereaction zone B is at least 10° C. or at least 20° C. above thetemperature of the heat exchange medium entering the reaction zone A.According to the invention, the difference between the temperatures atentry into the reaction zones A and B may thus be up to 15° C., up to25° C., up to 30° C., up to 35° C. or up to 40° C. However, theabovementioned temperature is usually not more than 50° C. The higherthe chosen acrolein loading of the catalyst bed in the novel process,the greater should be the difference between the temperature at whichthe heat exchange medium enters the reaction zone A and the temperatureat which the heat exchange medium enters the reaction zone B.Preferably, the temperature at entry into the reaction zone A is from245 to 260° C. and the temperature at entry into the reaction zone B isfrom 265 to 285° C.

In the novel process, the two reaction zones A, B can of course also berealized in tube-bundle reactors spatially separated from one another.If required, a heat exchanger may also be mounted between the tworeaction zones A, B. The two reaction zones A, B can of course also bedesigned as a fluidized bed.

Furthermore, in the novel process, it is also possible to use a catalystbed whose volume-specific activity in the direction of flow of thereaction gas mixture increases continuously, abruptly or stepwise (thiscan be achieved, for example, by dilution with inert material orvariation of the activity of the multimetal oxide).

For the two-zone procedure described, it is also possible to use theinert diluent gases recommended in EP-A 293224 and in EP-B 257565 (forexample, only propane or only methane, etc.). The latter can, ifrequired, also be combined with a volume-specific catalyst bed activitywhich decreases in the direction of flow of the reaction gas mixture.

It should once again be pointed out here that, for carrying out thenovel process, it is also possible to use in particular the two-zonetube-bundle reactor type which is described in DE-B 40 2201528 and whichincludes the possibility of diverting a portion of the relatively hotheat exchange medium of the reaction zone B to the reaction zone A inorder, if required, to heat up a cold reaction gas starting mixture or acold recycled gas.

The novel process is particularly suitable for a continuous procedure.It is surprising that it permits a high space-time yield in theformation of the desired product in a single pass without simultaneouslysignificantly impairing the selectivity of formation of the desiredproduct. Rather, there is generally an increase in the selectivity offormation of the desired product. The latter is presumably due to thefact that, owing to the higher temperatures present in the region of thehigher acrolein conversion, the novel process results in lessreadsorption of the resulting acrylic acid onto the fixed-bed catalyst.

Also noteworthy is the fact that the catalyst life in the novel processis completely satisfactory in spite of the extreme catalyst loading withreactants.

In the novel process, pure acrylic acid is not obtained, but instead amixture from whose secondary components the acrylic acid can beseparated off in a manner known per se (for example by rectificationand/or crystallization). Unconverted acrolein, propene and inert diluentgas used and/or formed in the course of the reaction can be recycled tothe gas-phase oxidation. In a two-stage gas-phase oxidation startingfrom propene, the recycling is expediently effected into the firstoxidation stage. The novel two-zone procedure can of course, ifrequired, also be used in the case of conventional propene loads.

Furthermore, unless mentioned otherwise, conversion, selectivity andresidence time are defined as follows in this publication:$\begin{matrix}{{\text{Conversion~~}{C_{A}\quad}\text{ofacrolein~~(\%)}} = \quad {\frac{\text{Number~~of~~moles~~of~~acroleinconverted}}{\text{Number~~of~~moles~~of~~acroleinused}} \times 100}} \\{{\text{Selectivity~~}S_{A}\text{~~ofthe~~acrylic~~acidformation~~(\%)}} = \quad {\frac{\text{Number~~of~~moles~~of~~acroleinconverted~~to~~acrylic~~acid}}{\text{Number~~of~~moles~~of~~acroleinconverted}} \times 100}} \\{\text{Residence~~time(sec)} = \quad {\frac{\text{Empty~~reactor~~volume~~filled~~withcatalyst~~(l)}}{\text{Throughput~~of~~reaction~~gasstarting~~mixture~~(l/h)}} \times 3600}}\end{matrix}$

EXAMPLES

a) Catalyst Preparation

1. Preparation of the Catalytically Active Oxide MaterialMo₁₂V₃W_(1.2)Cu_(2.4)O_(n)

190 g of copper(II) acetate monohydrate were dissolved in 2700 g ofwater to give a solution I. 860 g of ammonium heptamolybdatetetrahydrate, 143 g of ammonium metavanadate and 126 g of ammoniumparatungstate heptahydrate were dissolved in succession in 5500 g ofwater at 95° C. to give a solution II. The solution I was then stirredall at once into the solution II, after which a 25% strength by weightaqueous NH₃ solution was added in an amount sufficient to form asolution again. This was spray-dried at an outlet temperature of 110° C.The resulting spray-dried powder was kneaded with 0.25 kg, per kg ofpowder, of a 30% strength by weight aqueous acetic acid solution using aZS1-80 type kneader from Werner & Pfleiderer and then dried at 110° C.for 10 hours in a drying oven.

700 g of the catalyst precursor thus obtained were calcined in anair/nitrogen mixture [(200 l of N₂/15 l of air)/h] in a rotary tubularfurnace (50 cm long, 12 cm internal diameter).

During the calcination, the kneaded material was first continuouslyheated from room temperature (about 25° C.) to 325° C. in the course ofone hour. The temperature was then maintained for 4 hours. Thereafter,heating was effected to 400° C. in the course of 15 minutes, thistemperature was maintained for 1 hour and this was followed by coolingto room temperature.

The calcined catalytically active material was milled to give a finelydivided powder, 50% of the powder particles of which passed through asieve of mesh size from 1 to 10 μm and whose particle fraction having amaximum dimension above 50 μm was less than 1%.

b) Preparation of a Coated Catalyst

28 kg of annular carriers (7 mm external diameter, 3 mm length, 4 mminternal diameter, steatite, having a surface roughness Rz according toEP-B 714700 of 45 μm and a total pore volume, based on the volume of thecarriers, of ≦1% by volume, manufacturer: Caramtec DE) were introducedinto a 200 l coating pan (angle of inclination 90°; Hicoater fromLödige, DE). The coating pan was then rotated at 16 rpm. 2000 g of anaqueous solution consisting of 75% by weight of H₂O and 25% by weight ofglycerol were sprayed via a nozzle onto the carriers in the course of 25minutes. At the same time, 7 kg of the catalytically active oxide powderfrom a) were simultaneously metered in continuously in the same periodvia a vibratory conveyor outside the spray cone of the atomizer nozzle.During the coating, the powder fed in was completely absorbed onto thesurface of the carriers and no agglomeration of the finely dividedactive oxide material was observed. After the end of the addition ofpowder and aqueous solution, hot air at 110° C. was blown into thecoating pan for 20 minutes at a speed of 2 rpm. Drying was then carriedout for a further 2 hours at 250° C. in the stationary bed (tray oven)under air. Annular coated catalysts whose content of active oxidematerial was 20% by weight, based on the total mass, were obtained. Thecoat thickness was 230±25 μm both over the surface of a carrier and overthe surface of different carriers.

b) Gas-phase Catalytic Oxidation of Acrolein to Acrylic Acid

1. Loading the Reaction Tube

A V2A steel reaction tube having an external diameter of 30 mm, a wallthickness of 2 mm, an internal diameter of 26 mm and a length of 439 cmand having a thermocouple tube (4 mm external diameter) centered in themiddle of the reaction tube for receiving a thermocouple with which thetemperature in the reaction tube can be determined was loaded from thebottom upward on a catalyst support ledge (44 cm length) first withsteatite beads having a rough surface (from 4 to 5 mm diameter, inertmaterial for heating the reaction gas starting mixture) over a length of30 cm and then with the coated catalyst rings prepared in a) over alength of 300 cm, before the loading was completed with theabovementioned steatite beads as a subsequent bed over a length of 30cm. The remaining 35 cm of catalyst tube were left empty.

That part of the reaction tube which was loaded with solid wasthermostatted by means of 12 aluminum blocks cast cylindrically aroundthe tube and each having a length of 30 cm (comparative experimentsusing a corresponding reaction tube heated by means of a salt baththrough which nitrogen was bubbled showed that thermostatting by meansof an aluminum block was capable of simulating thermostatting by meansof a salt bath). The first six aluminum blocks in the direction of flowdefined a reaction zone A and the remaining six aluminum blocks defineda reaction zone B. The ends of the reaction tube which were free ofsolid were kept at 220° C. by means of steam under pressure.

The reaction tube described above was loaded continuously with areaction gas starting mixture of the following composition, the loadingand the thermostatting of the reaction tube being varied:

5.5% by volume of acrolein,

0.3% by volume of propene,

6.0% by volume of molecular oxygen,

0.4% by volume of CO,

0.8% by volume of CO₂,

9.0% by volume of water and

78.0% by volume of molecular nitrogen.

A small sample was taken from the product gas mixture at the reactiontube outlet for gas chromatographic analysis. An analysis point waslikewise present at the end of the reaction zone A.

The results obtained as a function of the chosen acrolein loading and ofthe chosen aluminum thermostatting are shown in Table 1 below.

T_(A) is the temperature of the aluminum blocks in the reaction zone Aand T_(B) is the temperature of the aluminum blocks in the reaction zoneB.

C_(AA) is the acrolein conversion at the end of the reaction zone A andC_(AE) is the acrolein conversion at the reaction tube outlet. S_(AE) isthe selectivity of the acrylic acid formation at the reaction tubeoutlet and STY_(A) is the space-time yield of acrylic acid at thereaction tube outlet.

Finally, it may be stated, that, instead of the catalyst bed used in theexample, a corresponding bed according to Example 3 of DE-A 19736105 mayalso be used.

TABLE 1 Acrolein loading [l (s.t.p.) of acro- lein/l · h] T_(A) [° C.]T_(B) [° C.] C_(AA) (%) C_(AE) (%) S_(AE) (%) STY_(A) (g/l · h)  87 255255 91.5 99.2 95.7 265 113 262 262 91.7 99.3 95.3 345 150 267 267 93.299.3 95.0 452 150 254 271 76.1 99.3 95.8 457 171 255 276 73.2 99.3 95.7523

If the acrolein load is increased to >175 l (s.t.p.) of acrolein/l.h,the results according to Table 2 are obtained.

TABLE 2 Acrolein loading [l (s.t.p.) of acro- lein/l · h] T_(A) [° C.]T_(B) [° C.] C_(AA) (%) C_(AE) (%) S_(AE) (%) STY_(A) (g/l · h) 190 257281 78.2 99.3 95.7 579 210 257 286 71.7 99.3 95.6 640

We claim:
 1. A process for the catalytic gas-phase oxidation of acroleinto acrylic acid, in which a reaction gas starting mixture comprisingacrolein, molecular oxygen and at least one inert gas, at least 20% byvolume of which consists of molecular nitrogen, and containing themolecular oxygen and the acrolein in a molar ratio O₂:C₃H₄O≧0.5 ispassed, at elevated temperatures, over a fixed-bed catalyst, whoseactive material is at least one molybdenum- and vanadium-containingmultimetal oxide, in such a way that the acrolein conversion in a singlepass is ≧90 mol % and the associated selectivity of the acrylic acidformation is ≧90 mol %, wherein a) the loading of the fixed-bed catalystwhere the acrolein contained in the reaction gas starting mixture is≧150 l (s.t.p.) of acrolein per l of catalyst bed per h, b) thefixed-bed catalyst consists of a catalyst bed arranged in two spatiallysuccessive reaction zones A, B, the temperature of the reaction zone Abeing from 230 to 270° C. and the temperature of the reaction zone Bbeing from 250 to 300° C. and at the same time being at least 5° C.above the temperature of the reaction zone A, c) the reaction gasstarting mixture flows first through the reaction zone A and thenthrough the reaction zone B and d) the reaction zone A extends to anacrolein conversion of from 55 to 85 mol %.
 2. A process as claimed inclaim 1, wherein the reaction zone A extends to an acrolein conversionof from 65 to 80 mol %.
 3. A process as claimed in claim 1, wherein thetemperature of the reaction zone B is at least 20° C. above that of thereaction zone A.
 4. A process as claimed in claim 1, wherein thetemperature of the reaction zone A is from 245 to 260° C.
 5. A processas claimed in claim 1, wherein the temperature of the reaction zone B isfrom 265 to 285° C.
 6. A process as claimed in claim 1, wherein theacrolein conversion in a single pass is ≧94 mol %.
 7. A process asclaimed in claim 1, wherein the selectivity of the acrylic acidformation is ≧94 mol %.
 8. A process as claimed in claim 1, wherein theacrolein loading of the catalyst bed is ≧160 l (s.t.p.)/l.h.
 9. Aprocess as claimed in claim 1, wherein the acrolein loading of thecatalyst bed is ≧170 l (s.t.p.)/l.h.
 10. A process as claimed in claim1, wherein the one or more inert gases comprise ≧40% by volume ofmolecular nitrogen.
 11. A process as claimed in claim 1, wherein the oneor more inert gases comprise steam.
 12. A process as claimed in claim 1,wherein the one or more inert gases comprise CO₂ and/or CO.
 13. Aprocess as claimed in claim 1, which is carried out at an operatingpressure of from 0.5 to 3 bar.
 14. A process as claimed in claim 1,wherein the molar O₂:acrolein ratio in the reaction gas starting mixtureis from 1 to
 2. 15. A process as claimed in claim 1, wherein air isconcomitantly used as an oxygen source.
 16. A process as claimed inclaim 1, wherein the acrolein content of the reaction gas startingmixture is from 3 to 15% by volume.
 17. A process as claimed in claim 1,wherein the acrolein content of the reaction gas starting mixture isfrom 5 to 8% by volume.
 18. A process as claimed in claim 1, wherein theactive material of the fixed-bed catalyst is at least one multimetaloxide of the formula I Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶_(g)O_(n)  (I) where X¹ is W, Nb, Ta, Cr and/or Ce, X² is Cu, Ni, Co,Fe, Mn and/or Zn, X³ is Sb and/or Bi, X⁴ is one or more alkali metals,X⁵ is one or more alkaline earth metals, X⁶ is Si, Al, Ti and/or Zr, ais from 1 to 6, b is from 0.2 to 4, c is from 0.5 to 18, d is from 0 to40, e is from 0 to 2, f is from 0 to 4, g is from 0 to 40 and n is anumber which is determined by the valency and frequency of the elementsin I other than oxygen.
 19. A process as claimed in claim 1, wherein theactive material of the fixed-bed catalyst is at least one multimetaloxide of the formula II [D]_(p)[E]_(q)  (II), where D is Mo₁₂V_(a″)Z¹_(b″)Z² _(c″)Z³ _(d″)Z⁴ _(e″)Z⁵ _(f″)Z⁶ _(g″)O_(x″), E is Z⁷₁₂Cu_(h″)H_(i″)O_(y″), Z¹ is W, Nb, Ta, Cr, and/or Ce, Z² is Cu, Ni, Co,Fe, Mn and/or Zn, Z³ is Sb and/or Bi, Z⁴ is Li, Na, K, Rb, Cs and/or HZ⁵ is Mg, Co, Sr and/or Ba, Z⁶ is Si, Al, Ti and/or Zr, Z⁷ is Mo, W, V,Nb and/or Ta, a″ is from 1 to 8 b″ is from 0.2 to 5, c″ is from 0 to 23,d″ is from 0 to 50, e″ is from 0 to 2, f″ is from 0 to 5, g″ is from 0to 50, h″ is from 4 to 30, i″ is from 0 to 20 and x″, y″ are numberswhich are determined by the valency and frequency of the elements in IIother than oxygen and p, q are numbers other than zero, whose ratio p/qis from 160:1 to 1:1, which is obtainable by separately preforming amultimetal oxide material (E) Z⁷ ₁₂Cu_(h″)H_(i″)O_(y″)  (E), in finelydivided form (starting material 1) and then incorporating the preformedsolid starting material 1 into an aqueous solution, an aqueoussuspension or a finely divided dry blend of sources of the elements Mo,V, Z¹, Z², Z³, Z⁴, Z⁵ and Z⁶, which contains the abovementioned elementsin the stoichiometry D Mo₁₂V_(a″)Z¹ _(b″)Z² _(c″)Z³ _(d″)Z⁴ _(c″)Z⁵_(f″)Z⁶ _(g″)  (D) (starting material 2), in the desired ratio p:q,drying any resulting aqueous mixture and calcining the resulting dryprecursor material before or after its drying to the desired catalystgeometry at from 250 to 600° C.
 20. A process as claimed in claim 1,wherein the catalyst bed comprises annular catalysts.
 21. A process asclaimed in claim 1, wherein the catalyst bed comprises sphericalcatalysts.
 22. A process as claimed in claim 1, which is carried out ina two-zone tube-bundle reactor.